Add an option to use a custom configuration file for kubectl

[zostay]

A recurring problem is multiple context support. However, each cluster only needs one context configuration. The kubectl command does not provide a way, as far as I can determine, to select context via environment, which would allow genifest to easily select the context for duration of working with a cluster. However, it does provide a KUBECONFIG environment for selecting the configuration. So, I think the solution is to add a kubeconfig option to the cluster configuration and then set that environment variable to that file. Then, each project can have a standard location for the file (preferably locked out from commit via .gitignore).